Ion beams have been used as probes in TOF (Time-of-Flight) mass spectroscopy of surfaces of material for years (Hammond et al., 1995). Imaging and elemental analysis by energy analysis of the backscattered ions and backscattered neutrals and forward recoiled elemental ions and neutrals created during the collisions between incident ions and surfaces can yield information on both surface structure and composition. Moreover, by scanning a Focused Ion Beams (FIB) and recording the secondary particle intensity as a function of beam location, images have been obtained of surfaces as diverse as semiconductor and biological surfaces. Secondary particles such as secondary electrons, secondary ions sputtered from the surface, and (less so) backscatter ions and neutrals have all been employed. The fundamental limitation preventing FIB imaging being as useful as secondary electron microscopy is only partly due to the inability to focus the ion microprobe to nanometer dimensions. More significant is the difficulty to capture a significant fraction of the diffuse secondary particle emission released during ion bombardment of the surface. Analyzing the energy and masses of these secondary particles with sufficient resolution in a time scale fast enough for rapid FIB surface imaging is yet another.
An example of these limitations of the prior art is to be seen for the specific case of ion (and neutral) backscattering and surface recoil analysis; however the limitation in this area which we will now discuss also transfer to the use of other secondary species for obtaining image contrast during microprobe surface imaging. Measuring the energy loss of the backscattered primary ions/neutrals generated when an KeV ion beam strikes the surface of a material has been used extensively for elemental and structural analysis of the surface for the last 25 years. The Co-Axial Impact Collision Ion Scattering Spectrometry (CAICISS) (Katayama et al., 1988 and Aono et al., 1992) technique (FIG. 1) is a special case of low energy Rutherford Backscattering which measures energy losses of backscattering Helium atoms and Helium ions when a nanosecond pulsed Helium ion beam impinges a surface. The energies of the backscattered Helium are determined by measuring their time of flight from the sample to the detector. The backscatter time of flight from the sample to the detector is relative to the time at which the Helium ion beam is initially pulsed. Since the mass of Helium is known and the length and geometry from the ion source to the sample and the sample to the detector are well-defined, the energy loss of each Helium atom arriving at the detector can be computed. The energy will be high (fast time of flight) when the Helium backscatters from a heavy element and low when it strikes a light element (slow time of flight). It is important to note when using ion scattering spectrometries—most primary ions neutralize as they closely approach the surface and remain neutralized as the primary particles backscatter from the surface. However, the velocity of Helium at a few hundred eV is still large enough to generate a substantial signal on a time of flight particle detector; therefore, the neutral helium can be detected and its time of flight measured. Thus most of the Helium which backscatters from the surface into an angle subtended by a backscatter particle detector can be used irrespective of its charge state.
We will now discuss other limits, some fundamental and some technical, to the elemental mass specificity of backscattering techniques. For example the physical scale of these instruments is a huge drawback. The beam-line and backscattering detector are over a meter in length. The actual flight path for the backscattered ions/neutrals is about 500 millimeters (mm); this path length is necessary to obtain an acceptable spectrum when the pulse duration of Helium is tens of nanoseconds. As seen in FIG. 1 (prior art) and FIG. 2 (illustrating the prior art juxtaposed against an embodiment of the present invention), the angle subtended by a 40 millimeters diameter detector is very small (approximately a two degree half angle) because of this large geometry. A further drawback to imaging is the rather large (a few hundred micron diameter) spatial focus of the ion beam on the sample.
Fundamental limitations also exist for detecting light elements, such as Oxygen. Light elements are detected poorly by Helium backscatter relative to heavier elements, such as Zinc (FIG. 3). This is because of higher scattering cross-sections of He from Zn compared to O and also because the He scattering from O is at a longer time which occurs on the straggling tail of the more intense He scatter from Zn. FIG. 3 shows two overlapped spectra from 2 keV Helium backscattering from a ZnO single crystal (Aono et al., 1992). In spectrum (b) the sample surface was tilted by 68 degrees from the Helium beam. The Helium scattered from Zinc is well resolved but no signal from Oxygen is observed. By contrast in spectrum (a) when the beam impinges the surface at 0 degrees incidence (normal to the surface), the backscatter from Zinc is no longer well resolved since a direct hit by the primary Helium onto the Zinc is blocked by a surface Oxygen. However Helium scatter from the surface Oxygen is not seen in either (a) or (b), simply because the cross-section for Helium backscatter decreases is significantly less than from Zn. It is thus desirable to combine backscattering with forward recoil detection by placing one or more detectors in the forward scattering direction so that the energy of light recoiled surface elements can be simultaneously determined.
Another example and application of CAICISS is monitoring film growth. However, elements which are close in mass such as Lanthanum (La) and Strontium (Sr) are difficult to resolve by backscatter due to nearly equal Helium backscatter flight times from each. Moreover, depending on the azimuthal scattering angle (angle by which the surface is rotated around its normal), the scattering signal intensities can vary significantly. The variation in signal intensity depends on scattering from heavy species like La or Sr compared to the lighter material like Manganese. Also, the variation in signal intensity depends on the surface structure (where each element is shadowing and blocking its nearest neighbor at certain angles). FIG. 4A shows the Helium Time-of-Flight backscatter spectra obtained with the beam incident at 55° (elevation above the surface plane is 35°) to determine the geometric structure of the surface by the use of backscattered Helium (Ohnishi et al., 1998). The dependence of the scattered Helium intensity on the azimuthal angle is shown in FIG. 4B. The signal intensities can vary significantly depending on the azimuthal scattering angle, the scattering from heavy species like La or Sr versus lighter materials like Manganese and depending on the surface structure (where each element is shadowing and blocking its nearest neighbor at certain angles). The azimuthal angle and/or elevation scanning (rotation and/or tilting) of the sample relative to the Helium beam incidence plane can clearly be used to provide information regarding local surface crystallography and these techniques yield local geometries which cannot be measured by more long-wavelength diffraction techniques such as electron or x-ray diffraction. However, the practical use of this technique is limited by the time necessary to turn the sample and record intensity variations into a small angular acceptance backscatter detector. While this problem has been partly addressed in the prior art by using large acceptance angle position sensitive detectors which reduces the need for some of the sample adjustments, such devices still remain relatively large, slow, and cumbersome.
The variance in backscattering intensities as a function of atomic number (Z) in (FIGS. 4A and 4B) would be lower from this sample if a Neon primary ion beam were used since the overall variance of Neon backscatter cross-sections is less as a function of atomic number (Z); however, Neon cannot backscatter from any element lighter than itself, which precludes any backscattering from first row elements such as Fluorine. However, the lighter elements are efficiently forward recoiled by the Neon towards the surface. The forward recoiled lighter elements then scatter backwards and/or sideways from their heavier nearest neighbors and the lighter elements arrive at the backscatter detector with keV type energies and flight times which are faster than the backscattered Neon. However, not much practical use has been made of this phenomenon other than to study the essential physics of the multiple atom collision sequences involved. An alternative has been to tilt the sample relative to the incident ion beam and to position a position sensitive detector to intercept forward recoiled surface atoms and ions and forward scattered primary particles.
Another aspect involves Secondary Ion Mass Spectrometry (SIMS) imaging of surfaces, combined with secondary electron detection and ion scattering. Spatially resolved microprobe images of the surface are routinely obtained by measuring and recording the variation of the secondary electron yield as a micro-focused energetic primary particle beam (such as an electron, ion, photon is scanned from one micro-focused point on the surface to the next. While the previous discussion focused on combining ion backscattering experiments into an ion microprobe which also can image a surface by detecting secondary electrons, it is well known that other ejection processes simultaneously occur when a focused energetic primary particle beam (photon, electron, ion) strikes the surface. Extremely useful elemental and molecular images of the surface may be obtained by simultaneously using other contrast mechanisms to augment the secondary electron images. For example other images can be obtained using the intensity, the energy and/or the mass of secondary ejected particles. The secondary ejected particles include but are not limited to photons, backscattered primary particles, secondary ions directly created and sputtered by the incoming primary particle beam, or secondary ions created by photoionizing secondary sputtered neutral elements or molecules. The sputtering of secondary neutrals is often the most predominant sputtering channel for many elements and molecules on the surface. Focused ion beams have been also used with SIMS for elemental and molecular analysis and imaging of these surfaces using magnetic, time-of flight, or orthogonal time of flight mass spectrometers; however prior art mass spectrometers are necessarily large in order to obtain the high mass resolution necessary to identify secondary elemental ions from secondary molecular ions which directly conflicts with the needs of microprobe imaging for fast scanning from one micropixel to the next on the surface in order to obtain the image in the shortest time possible. The need for large, high mass resolution mass spectrometers with bulky secondary ion extraction optics also conflicts directly with the necessity for the micro-focusing primary beam optics to also be as close as possible to the sample. The long flight time of secondary particles through large secondary particle analysis instrumentation is an ever-present conflict with the stringent requirements of surface imaging—namely, during imaging it is imperative to quickly move the position of the micro-focused primary particle from one surface location to the next while recording the intensity of the secondary particles as quickly as possible.
The present invention provides a detector suite for correlating some or all such coincident and non co-incident secondary particle emissions to simultaneously obtain primary particle beam microprobe spatial imaging. Each of the different types of secondary particles is detected either singly or in parallel and their intensities and co-incidences are recorded as the focused microprobe is scanned over the surface from one location to the next.